Missense mutations in the presenilin (PS1/PS2) genes are responsible for greater than 50% of case of early-onset familial Alzheimer's disease (FAD). Neither the normative functions nor the mechanisms by which PS1/PS2 variants predispose individuals to the pathological/clinical phenotype of AD have been defined. However, recent studies revealed that PS1/PS2 are homologous to sel-12 C. elegans protein involved in cell signaling pathways mediated by lin-12/Notch receptors that specify cell fate. Building on our longstanding interest and expertise in the clarification of issue relevant to the processing/ expression of amyloid precursor proteins (APP) and the pathogenesis of AD, we propose: to examine the expression of PS1/PS2 in brain during development and aging; to define the topology and metabolism of PS1/PS2 in cultured mammalian cells; and to examine the influences of PS1 and PS1 variant expression on APP/A beta (Beta-amyloid protein) trafficking and metabolism in cultured cells and primary neurons of transgenic mice that express human APP and PS1. To define the levels, cellular specificities, and the subcellular distributions of PS1/PS2 in cultured cells as well as in rodent, monkey and human brains, we will generate a battery of nucleic acid probes for in situ hybridization and RT- PCR studies and monoclonal/polyclonal antibodies to epitopes specific for each polypeptide. Studies of PS1/PS2 topology are critical for establishing the potential structure/function relationships of domains in these molecules and may reveal important features of the effects of FAD- linked mutations on PS function. Finally, to test the recent suggestion that PS1 variants influence the amyloidogenic A beta processing of APP, we will assess, using transfection strategies, the influence of PS1 or PS1 variant expression on APP trafficking and A beta production. These latter issue will also be addressed in primary neuronal cultures of transgenic animals that express human APP and PS1 regarding PS1/PS2 expression in vivo, the normative biology of these polypeptides, and the influences of PS on APP/A beta metabolism.